The results tend to support the theory, which reasons that the degree of seller interdependency, as measured by concentration levels, is inversely related to the strength of competition in a market. While not universal, the results are impressive in light of the variety of time periods, profit and concentration measures, and industry samples used in the different research projects.

This book is designed to address policy questions concerning the preservation and promotion of competition in the domestic production of primary energy. While economic theory is essential to formulating public policy, Prof. Baumol has noted some inherent limitations of theory in this role. Theory, according to Baumol, is better suited to the task of recommending against, rather than supporting, a particular policy action. This results from the necessity to construct models which, by their nature, oversimplify reality. Baumol notes that

... since we have good reasons to suspect the representativeness of the model, we are not likely to have much confidence in policy proposals for which it serves as primary justification.

Thus, it is the nature of model building that weakens the ability of theory to take a positive position in formulating policy.

In spite of the weakness of theory, the available evidence linking market structure to performance has been sufficient to cause U.S. antitrust policy to take a clear turn toward a structural approach and away from a conduct approach. The net effect of this change in relative emphasis is almost certain to produce a much more balanced antitrust policy.